1. Field of the Invention
The invention relates to the feeding and transport of flexible sheet material and, more particularly, the feeding and transport of such material on a rotary drum.
2. Description of the Prior Art
Rotary drum transport mechanisms for sheet material appear to attempt to maintain a constant velocity of the drum, especially where processing of the sheet material is involved. Drums having high inertias are therefore often used to reduce the effect of high frequency disturbances on drum velocity.
Such drums are thus used for relatively slow rotational velocity applications where the entire processing of the sheet material is accomplished in one revolution or less and possibly uses only continuous sheet material.
The use of cut sheet material is advantageous in many situations and is required if the processing requires more than one revolution. Cut sheet material further requires mechanisms to feed each sheet onto the drum, register and attach the sheet, and strip the sheet from the drum. None of these steps is trivial. Witness U.S. Pat. No. 3,784,190, Crawford, assigned in common with the present invention, wherein a rotary drum and feeding, processing and stripping of cut sheet material are described.
A way to both increase processing speed and to reduce the machine size is to employ a rotary drum of smaller size and significantly higher speed. Feeding and attaching mechanisms of high to extreme complexity and tolerance are therefore required to accelerate the sheet material, register the sheet material on the rotating drum, attach the sheet material to the rotating drum, strip the processed material from the drum, and decelerate the material.
At the higher speeds, attaching and registering the sheet to the drum while holding the remainder of the sheet against the centrifugal force generated by the rotation becomes important. Vacuum has been employed to attach leading edges of sheet material and occasionally other parts of the sheet, such as the trailing edges. Electrostatic attraction has also been employed to hold insulative sheets, such as paper, to conductive drums at slow speeds, although it has normally been only a by-product of electrophotographic processing. For example, U.S. Pat. Nos. 3,506,259, Caldwell et al, and 3,804,401, Stange, describe mechanisms for separating sheets from a drum to which they are electrostatically attracted. Caldwell et al in particular presents the connotation that such electrostatic "tacking" is a problem. Crawford indicates that separation becomes far more difficult at higher speeds and proposes a solution to the problem.
It is therefore an object of the present invention to provide a sheet feeding, transport and unloading arrangement for a rotary drum which operates at high speed for processing the sheet material.